The present invention relates in general to digital radio receivers employing coherent (i.e., synchronous) detection, and more specifically to the generation of a coherent signal using adaptive notch filter techniques, without use of a phase-locked loop.
Coherent detection (i.e., synchronous detection) is a well-known technique employed in a variety of radio receivers, especially receivers for amplitude modulated signals. This type of demodulation requires that the receiver have a signal available with the same phase and frequency as the original modulating signal used prior to signal transmission. Coherent detection is used to demodulate mono and stereo commercial AM broadcasts and the stereo difference channel in commercial FM stereo broadcasts.
The coherent signal used in coherent detection is typically generated using a phase-locked loop (PLL). FM stereo broadcasts include the transmission of a pilot signal which is isolated by the receiver and input to the phase-locked loop. In AM receivers, the PLL locks to the carrier signal which is transmitted as the center frequency of the AM broadcast.
Although commercial AM and FM broadcasts are comprised of a modulated analog signal, it is becoming increasingly popular to convert the received analog signal into a digital signal and to process the radio broadcast signal digitally. Such digital signal processing (DSP) receivers realize various advantages, such as circuit integration, reduced size, exact operation, minimal adjustments, and the ability to combine signal processing for various audio functions (e.g., tone control, concert hall emulation, and equalization).
It is well-known that digital systems must employ a sample rate equal to at least twice the frequency of the analog signal to be represented. Thus, a digital sampling of FM broadcast signals at their transmitted radio frequencies of over 100 MHz would require sample rates in excess of 200 MHz, which is undesirable. Instead, it is desirable to employ an analog tuner to generate a signal at a lower frequency (e.g., a demodulated signal or an intermediate frequency signal) which can be represented using a lower sample rate. For example, a standard analog tuner is employed to generate an FM intermediate frequency (IF) of 10.7 MHz which is FM detected to form an analog FM baseband signal. The baseband signal has a frequency range of from 50 Hz to 53 kHz and is digitized in an analog-to-digital converter (ADC). A pilot signal is isolated from the FM baseband signal using a digital bandpass filter. The pilot signal is then input into a digital phase-locked loop for generating a coherent digital signal.
Digital phase-locked loops suffer several disadvantages. As in any phase-locked loop, the requirement for a loop filter tends to slow down the locking process of the phase-locked loop and also limits the maximum capture range. Furthermore, a digital phase-locked loop employs excessive amounts of processing time and software code.